A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as carbon monoxide, sulfur oxides and nitrogen oxides in waste gas streams which result from the processing and combustion of sulfur, carbon and nitrogen containing fuels. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide, carbon monoxide and nitrogen oxide concentrations that are frequently encountered in conventional operations. The regeneration of cracking catalyst, which has been deactivated by coke deposits in the catalytic cracking of sulfur and nitrogen containing hydrocarbon feedstocks, is a typical example of a process which can result in a waste gas stream containing relatively high levels of carbon monoxide, sulfur and nitrogen oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated, transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes. Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air. The combustion of these coke deposits can be regarded, in a simplified manner, as the oxidation of carbon and the products are carbon monoxide and carbon dioxide.
When sulfur and nitrogen containing feedstocks are utilized in catalytic cracking process, the coke deposited on the catalyst contains sulfur and nitrogen. During regeneration of coked deactivated catalyst, the coke is burned from the catalyst surface that then results in the conversion of sulfur to sulfur oxides and nitrogen to nitrogen oxides.
The conditions experienced by the catalyst in a fluid catalytic cracking (FCC) unit are very severe. Catalyst is continuously being cycled between reductive atmosphere on the reactor side to an oxidative atmosphere on the regenerator side. The temperatures between the two zones are different so the catalyst experiences thermal shocks. Also the regenerator contains nominally about 15-25% steam. All these factors lead to a significant decline in the catalyst activity and fresh catalyst needs to be continuously added to maintain the cracking activity.
Various approaches have been used to either reduce the formation of noxious gases or treat them after they are formed. Most typically, additives have been used either as an integral part of the FCC catalyst particles or as separate particles in admixture with the FCC catalyst.
The additive that has gained the widest acceptance for lowering sulfur oxide emissions to date in FCC units (FCCU) is based upon Magnesium oxide/Magnesium aluminate/ceria technology. Pt supported on clay or alumina is most commonly used as an additive for lowering of carbon monoxide emissions. Unfortunately the additives used to control CO emissions typically cause a dramatic increase (e.g. >300%) in NOx evolution from the regenerator.
Various approaches have been used to treat nitric oxide gases in FCCU. For example, U.S. Pat. No. 5,037,538 describes the reduction of oxides of nitrogen (NOx) emissions from an FCC regenerator by adding a deNOx catalyst to the FCC regenerator in a form whereby the deNOx catalyst remains segregated within the FCC regenerator.
U.S. Pat. No. 5,085,762 describes the reduction of emissions of noxious nitrogen oxides with the flue gas from the regenerator of a fluid catalytic cracking plant by incorporating into the circulating inventory of cracking catalyst separate additive particles that contain a copper-loaded zeolite material having a characteristic structure with a defined X-ray diffraction pattern.
U.S. Pat. No. 5,002,654 describes a process for regeneration of cracking catalyst while minimizing NOx emissions using a zinc-based deNOx catalyst.
U.S. Pat. No. 5,021,146 describes a process for regeneration of cracking catalyst while minimizing NOx emissions using a Group IIIb based deNOx additive.
U.S. Pat. No. 5,364,517 and U.S. Pat. No. 5,364,517 describe the reduction of the NOx content of FCC regenerator flue gas is reduced using a spinel/perovskite additive.
U.S. Pat. No. 5,750,020 and U.S. Pat. No. 5,591,418 describe process for removing sulfur oxides or nitrogen oxides from a gaseous mixture in an FCC process using a collapsed composition which is substantially composed of microcrystallites collectively of the formula:M2m2+Al2·pMp3+TrO7+r·swhere M2+ is a divalent metal, M3+ is a trivalent metal, and T is vanadium, tungsten, or molybdenum.
U.S. Pat. No. 6,165,933 describes compositions comprising a component containing (i) an acidic oxide support, (ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) palladium; to promote CO combustion in FCC processes while minimizing the formation of NOx.
U.S. Pat. No. 6,129,834 and U.S. Pat. No. 6,143,167 describe compositions comprising a component containing (i) an acidic oxide support, (ii) an alkali metal and/or alkaline earth metal or mixtures thereof, (iii) a transition metal oxide having oxygen storage capability, and (iv) a transition metal selected from Groups Ib and/or IIb of the Periodic Table; to provide NOx control performance in FCC processes.
All the additives added to FCC units need to have sufficient hydrothermal stability to withstand the severe environment of an FCCU and there remains the need for NOx additives to be used in FCC that have improved hydrothermal stability.